The present invention relates to the compression of gas in a rotary compressor. More particularly, the present invention relates to the control, by the use of a gaseous medium, of the position of a slide valve in a refrigeration screw compressor.
Compressors are used in refrigeration systems to raise the pressure of a refrigerant gas from an evaporator to a condenser pressure (more generically referred to as suction and discharge pressures respectively) which permits the ultimate use of the refrigerant to cool a desired medium. Many types of compressors, including rotary screw compressors, are commonly used in such systems. Rotary screw compressors employ male and female rotors mounted for rotation in a working chamber which is a volume shaped as a pair of parallel intersecting flat-ended cylinders closely toleranced to the exterior dimensions and shapes of the intermeshed screw rotors.
A screw compressor has low and high pressure ends which respectively define suction and discharge ports that open into the working chamber. Refrigerant gas at suction pressure enters the suction port from a suction area at the low pressure end of the compressor and is delivered to a chevron shaped compression pocket formed between the intermeshed rotors and the interior wall of the working chamber.
As the rotors rotate, the compression pocket is closed off from the suction port and gas compression occurs as the pocket's volume decreases. The compression pocket is circumferentially and axially displaced to the high pressure end of the compressor where it comes into communication with the discharge port.
Screw compressors most typically employ slide valve arrangements by which the capacity of the compressor is controlled over a continuous operating range. The valve portion of a slide valve assembly is disposed within and constitutes a part of the rotor housing. Certain surfaces of the valve portion of the slide valve assembly cooperate with the rotor housing to define the working chamber of the compressor.
Slide valves are axially moveable to expose a portion of the working chamber and the rotors therein to a location within a screw compressor, other than the suction port, which is at suction pressure. As a slide valve opens to greater and greater degrees, a larger portion of the working chamber and the screw rotors therein are exposed to suction pressure other than through the suction port. The portion of the rotors and working chamber so exposed is prevented from engaging in the compression process and the compressor's capacity is proportionately reduced. The positioning of the slide valve between the extremes of the full load and unload positions is relatively easily controlled as is, therefore, the capacity of the compressor and the system in which it is employed. Historically, slide valves have been positioned hydraulically using oil which has a multiplicity of other uses within the compressor.
In refrigeration applications, such other uses of oil in a screw compressor include bearing lubrication and the injection of oil into the gas undergoing compression in the working chamber of the compressor. Injected oil acts as a sealant between the meshing screw rotors and between the rotors and the interior surface of the working chamber. The injected oil also lubricates and prevents excess wear between the rotors. Finally, in some applications oil is injected into the working chamber to cool the refrigerant undergoing compression which, in turn, reduces thermal expansion in the compressor and allows for tighter rotor clearances at the outset.
Such oil is most typically sourced from an oil separator where discharge pressure is used to drive oil from an oil sump in the separator to compressor injection ports and bearing surfaces and to control the position of the slide valve. In each case, the pressure differential between the relatively higher pressure source of the oil (the oil separator) and a location within the compressor which is at a relatively lower pressure is taken advantage of to ultimately return the oil, after its use, to the oil separator.
In that regard, oil which has been used for its intended purpose in a screw compressor is vented or drained from the location of its use to a relatively lower pressure location within the compressor or in the system in which the compressor is employed. In the typical case, such oil is vented or drained to, or is used in the first instance, in a location which contains refrigerant gas at suction pressure or at some pressure which is intermediate compressor suction and discharge pressure.
Such oil mixes with and becomes entrained in the refrigerant gas in the location to which it is vented, drained or used and is delivered back to the oil separator, at discharge pressure, in the stream of compressed refrigerant gas discharged from the compressor. The oil is separated from the refrigerant gas in the separator and is deposited in the sump therein from which it is directed, most often using the discharge pressure which exists in the oil separator, back to the compressor locations identified above for further use. Even after the occurrence of the separation process, however, the oil in the sump of the oil separator will contain refrigerant gas bubbles and/or quantities of dissolved refrigerant. The separated oil may, in fact, contain from 10-20% refrigerant by weight depending upon the solubility properties of the particular oil and refrigerant used.
One difficulty and disadvantage in the use of such oil to hydraulically position the slide valve in a screw compressor relates to the fact that the oil used for that purpose will, as noted above, typically contain at least some dissolved refrigerant and/or bubbles of refrigerant gas. As a result of the use of such fluid to hydraulically position the piston by which the compressor slide valve is actuated, slide valve response can sometimes be inconsistent, erratic and/or slide valve position can drift as dissolved refrigerant entrained in the hydraulic fluid vaporizes (so-called "out gassing") or as entrained refrigerant gas bubbles collapse.
The out-gassing of refrigerant from the hydraulic fluid, which occurs when the pressure in the cylinder in which the slide valve actuating piston is housed is vented to cause unloading of the compressor, and the collapse of refrigerant gas bubbles entrained therein causes a volumetric change in the hydraulic fluid which affects the ability of that fluid to maintain slide valve position or properly position the slide valve in the first instance. Further, under certain conditions, such as where ambient temperatures at compressor startup cause system pressures downstream of the compressor discharge port to be lower than the pressure of gas undergoing compression in the compressor's working chamber, the pressure in the oil separator may be insufficient to cause the slide valve to move or be sufficiently responsive for safe and reliable compressor operation.
Still another disadvantage in the use of oil to hydraulically position the slide valve in a refrigeration screw compressor relates to the fact that the quantity of refrigerant gas bubbles and dissolved liquid refrigerant contained therein varies with time and with the characteristics and composition of the particular batch of lubricant delivered to the slide valve actuating cylinder. In that regard, slide valves are most typically controlled through a supposition that the opening of a load or unload solenoid valve for a predetermined period of time results in slide valve movement that is repeatable and consistent with that period of time. That supposition is, in turn, predicated on the supposition that the characteristics and composition of the oil directed to or vented from the slide valve actuating cylinder during such a period of time is consistent.
However, because of the inconsistency in the characteristics and composition of the oil supplied to and vented from the slide valve actuation cylinder with respect to the nature and amount of refrigerant contained therein, slide valve movement during any particular time period is not precisely repeatable or predictable. This lack of consistency and repeatability, from the control standpoint, is disadvantageous and reduces the efficiency of the compressor.
The need therefore exists for an arrangement by which to control the position of a slide valve in a refrigeration screw compressor which eliminates the disadvantages associated with the use of hydraulic fluid in which dissolved refrigerant and/or refrigerant gas bubbles exist and which permits the more precise and consistent control of slide valve position under all compressor and system operating conditions including those during which downstream system pressure is less than the pressure which is reached in the compression pockets internal of the compressor's working chamber.